In general, a phase lock loop is a circuit that can be used to synchronize a variable local oscillator's signal with the phase of an intermediate frequency (IF) signal or, more broadly, a circuit that controls an oscillator or periodic frequency generator so as to operate it with a constant phase angle relative to a reference signal source.
A problem arises in attempting to phase lock a frequency-agile modulated output to the same frequency as that of a specific tooth of a comb generated output. In the prior art, a circuit had to be specifically designed and tuned to the desired comb tooth in order to generate incrementally related frequencies. This operation is difficult and expensive to do in a frequency-agile setting where it is desired to select from a plurality of comb frequencies.
For example, in one configuration, a comb generator (a circuit that produces discrete regularly spaced frequencies) may be used at the headend of a cable television system to produce an output frequency corresponding to each of a plurality of channels provided by the headend. One of two types of comb generators may be used--a harmonically related coherent (HRC) and an incrementally related coherent (IRC) comb generator. An HRC comb generator may provide a channel at 54 MHz, 60 MHz, 66 MHz, and so on up to the maximum channel frequency. An IRC comb generator is similarly operated, except that every channel is translated (shifted), up in frequency by 1.25 MHz (e.g., 55.25 MHz, 61.25 MHz, 67.25 MHz, etc.). The IRC comb generator provides a signal which is generated from two frequency components: a spacing frequency and a translating frequency. In general, these frequencies are not phase coherent with the headend's input video IF signal. Consequently, phase locking the video IF signal to the IRC comb generated frequency is made complicated because of the non-coherent phase of these two signals.
In a conventional fixed channel cable television system, a particular tooth of a comb generator is selected with a bandpass filter, as a function of the desired channel and frequency to be output from the headend. For example, to tune channel 3 at 61.25 MHz, a fixed channel converter comprises a bandpass filter which is tuned to that particular comb channel. In order to tune different channels, the bandpass filter has to be changed. In the prior art, the complete module which contains the filter has to be changed.
In a known configuration, a filtered comb tooth is mixed with a desired output channel local oscillator to produce a 45.75 MHz component. This component is phase locked to the modulated IF signal by varying the local oscillator frequency. This known system, the Scientific Atlanta Model 6350, is a CATV quality fixed channel modulator. In the cable television business, it is desirable to replace fixed channel units with frequency-agile units in order to achieve operational flexibility when using either a HRC or IRC generator.
Some cable television headends presently phase lock each channel's output signal to a particular tooth in a single comb generator. These comb generators produce a plurality of equally spaced operating frequencies (i.e., for IRC, 55.25 MHz, 61.25 MHz, 67.25 MHz, etc.). In most fixed channel output frequency applications, an appropriate tooth corresponding to a desired output channel frequency is used to provide the phase lock signal. The IRC comb generator is particularly difficult to phase lock in a frequency-agile situation, because the component signals frequencies are non-coherent with respect to the IF signal frequency. Consequently, when a frequency-agile up/down converter is used for these headend operations and each time a new output frequency is desired, the filter used to select a proper comb tooth has to be changed thereby negating most of the benefits of a frequency-agile system.
The problem occurs for these headends in using an IRC comb generator, because of the two frequencies used to generate the comb signals. FIG. 1 shows the component signals of a typical IRC comb generator. The first component, Fs, is the frequency of the normal spacing between channels (usually 6 MHz). The second component, Ft, is the translation frequency (e.g., 277.25 MHz). Both of these components must be used in order to accurately synchronize the output frequency of the up/down converter with the respective tooth of the IRC comb generator.